Raise an exception if using amp_level with native amp_backend

CHANGELOG.md

@@ -246,6 +246,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Changed `HorovodPlugin.all_gather` to return a `torch.Tensor` instead of a list ([#9696](https://github.com/PyTorchLightning/pytorch-lightning/pull/9696))
 
 
+- Raise an exception if using `amp_level` with native `amp_backend` ([#9755](https://github.com/PyTorchLightning/pytorch-lightning/pull/9755))
+
+
 ### Deprecated
 
 - Deprecated `LightningModule.summarize()` in favor of `pytorch_lightning.utilities.model_summary.summarize()`

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -563,11 +563,18 @@ def select_precision_plugin(self) -> PrecisionPlugin:
                 return TPUHalfPrecisionPlugin()
 
             if self.amp_type == AMPType.NATIVE:
+                if self.amp_level is not None:
+                    raise MisconfigurationException(
+                        f"You have asked for amp_level={self.amp_level} which is not supported "
+                        "with amp_backend='native'."
+                    )
+
                 log.info(f"Using native {self.precision} bit Automatic Mixed Precision")
                 if self._is_sharded_training_type:
                     return ShardedNativeMixedPrecisionPlugin(self.precision, use_cpu=self.use_cpu)
                 if self._is_fully_sharded_training_type:
                     return FullyShardedNativeMixedPrecisionPlugin(self.precision, use_cpu=self.use_cpu)
+
                 return NativeMixedPrecisionPlugin(self.precision, use_cpu=self.use_cpu)
 
             if self.amp_type == AMPType.APEX:
@@ -581,6 +588,10 @@ def select_precision_plugin(self) -> PrecisionPlugin:
                         "Sharded Plugin is not supported with Apex AMP, please using native AMP for 16-bit precision."
                     )
                 log.info("Using APEX 16bit precision.")
+
+                if self.amp_level is None:
+                    self.amp_level = "O2"
+
                 return ApexMixedPrecisionPlugin(self.amp_level)
 
         raise MisconfigurationException(

pytorch_lightning/trainer/trainer.py

@@ -172,7 +172,7 @@ def __init__(
         prepare_data_per_node: Optional[bool] = None,
         plugins: Optional[Union[PLUGIN_INPUT, List[PLUGIN_INPUT]]] = None,
         amp_backend: str = "native",
-        amp_level: str = "O2",
+        amp_level: Optional[str] = None,
         distributed_backend: Optional[str] = None,
         move_metrics_to_cpu: bool = False,
         multiple_trainloader_mode: str = "max_size_cycle",
@@ -190,7 +190,8 @@ def __init__(
 
             amp_backend: The mixed precision backend to use ("native" or "apex").
 
-            amp_level: The optimization level to use (O1, O2, etc...).
+            amp_level: The optimization level to use (O1, O2, etc...). By default it will be set to "O2"
+                if ``amp_backend`` is set to "apex".
 
             auto_lr_find: If set to True, will make trainer.tune() run a learning rate finder,
                 trying to optimize initial learning for faster convergence. trainer.tune() method will
@@ -203,7 +204,7 @@ def __init__(
                 Additionally, can be set to either `power` that estimates the batch size through
                 a power search or `binsearch` that estimates the batch size through a binary search.
 
-            auto_select_gpus: If enabled and `gpus` is an integer, pick available
+            auto_select_gpus: If enabled and ``gpus`` is an integer, pick available
                 gpus automatically. This is especially useful when
                 GPUs are configured to be in "exclusive mode", such
                 that only one process at a time can access them.
@@ -228,7 +229,7 @@ def __init__(
             devices: Will be mapped to either `gpus`, `tpu_cores`, `num_processes` or `ipus`,
                 based on the accelerator type.
 
-            distributed_backend: Deprecated. Please use 'accelerator'.
+            distributed_backend: Deprecated. Please use ``accelerator``.
 
             fast_dev_run: Runs n if set to ``n`` (int) else 1 if set to ``True`` batch(es)
                 of train, val and test to find any bugs (ie: a sort of unit test).

tests/plugins/test_amp_plugins.py

@@ -224,3 +224,9 @@ def test_cpu_amp_precision_16_throws_error(tmpdir):
             default_root_dir=tmpdir,
             precision=16,
         )
+
+
+@RunIf(min_gpus=1, amp_native=True)
+def test_amp_level_raises_error_with_native(tmpdir):
+    with pytest.raises(MisconfigurationException, match="not supported with amp_backend='native'"):
+        _ = Trainer(default_root_dir=tmpdir, gpus=1, amp_level="O2", amp_backend="native", precision=16)